1. Field of Invention
This invention pertains to error signal generation and servo systems usable for positional control of optical elements in optical systems. More particularly, the invention pertains to an error signal generation system for continuous, accurate positional control of a tunable element with respect to a coherent light beam.
2. Description of the Background Art
Fiberoptic telecommunications are continually subject to demand for increased bandwidth. One way that bandwidth expansion has been accomplished is through wavelength division multiplexing (WDM) wherein multiple separate data streams exist concurrently in a single optical fiber, with modulation of each data stream occurring on a different channel. Each data stream is modulated onto the output beam of a corresponding semiconductor transmitter laser operating at a specific channel wavelength, and the modulated outputs from the semiconductor lasers are combined onto a single fiber for transmission in their respective channels. The International Telecommunications Union (ITU) presently require channel separations of approximately 0.4 nanometers, or about 50 GHz. This channel separation allows up to 128 channels to be carried by a single fiber within the bandwidth range of currently available fibers and fiber amplifiers. Improvements in fiber technology together with the ever-increasing demand for greater bandwidth will likely result in smaller channel separation in the future.
Transmitter lasers used in WDM systems have typically been based on distributed feedback (DFB) lasers operating with a reference etalon associated in a feedback control loop, with the reference etalon defining the ITU wavelength grid. Statistical variation associated with the manufacture of individual DFB lasers results in a distribution of channel center wavelengths across the wavelength grid, and thus individual DFB transmitters are usable only for a single channel or a small number of adjacent channels. Continuously tunable external cavity lasers have been developed to overcome this problem.
The trend towards smaller channel separation and the advent of channel selectivity in transmitter lasers has given rise to a need for greater accuracy and control in the positioning of tunable elements associated with transmitter lasers. As tunable elements are configured for narrower channel separation, decreasing component tolerances and thermal fluctuation become increasingly important. Non-optimal positioning of tunable elements results in spatial losses and reduced transmitter output power.
The present invention relates to an error signal generation system and method for continuous and accurate tuning of a tunable element used in association with a coherent light source. In its most general terms, the invention comprises a coherent light beam with a fixed frequency or wavelength, a tunable element positioned in the light beam, and a detector, positioned in association with the light beam and tunable element, that is capable of generating an error signal indicative of a characteristic or property associated with the relationship of the element and the light beam. The invention also may comprise a tuning assembly operatively coupled to the tunable element and detector and configured to tune the tunable element according to the error signal generated by the detector.
The tunable element defines at least one characteristic or property with respect to the coherent light beam which is adjustable according to the error signal derived from the detector. For example, the tunable element may define a constructive interference fringe for the coherent light beam, with non-optimal tuning of the tunable element resulting in the constructive interference fringe being non-centered with respect to the light beam, resulting in spatial losses to the beam as it is transmitted through or reflected off the tunable element. The detector is positioned to detect such spatial losses and generate a corresponding error signal, which is usable to re-center the constructive interference fringe in the coherent light beam.
In certain embodiments the detector is a split detector, and non-optimal tuning of the tunable element such that the constructive interference fringe is not centered within the light beam will result in unequal amounts of optical power being detected by the two halves of the split detector. The voltage outputs of the detector halves are utilized to generate an error signal which corresponds to or is indicative of the optical power received by the different halves of the split detector. The error signal generated by the detector is used to tune the tunable element by adjusting a property of the tunable element by the tuning assembly so that the constructive interference fringe defined by the tunable element remains centered in the coherent light beam. In other embodiments the detector may comprise various types of multi-element detector, or a lateral effect detector.
The tunable element, in one embodiment, comprises a tapered or xe2x80x9cwedgexe2x80x9d etalon which may, for example, be in the form of an air gap between reflective surfaces of adjacent substrates, a single, solid substrate with tapered reflective surfaces, or a tapered thin film interference filter. The wedge etalon defines a constructive interference fringe for the wavelength of the coherent light beam. The tuning assembly may comprise a mechanical, electrical, piezoelectric or like system configured drive or translate the wedge etalon with respect to the light beam, or to apply a voltage, magnetic field, mechanical stress, or other effect which alters the characteristics of the wedge etalon. The tuning assembly may comprise, for example, a stepper motor that is configured to translate the wedge etalon such that the constructive interference fringe is moved with respect to the center of light beam.
The tunable element of the invention may alternatively comprise an air gap etalon embodied in a micro electrical mechanical system (MEMS) device wherein the air gap etalon is defined by parallel, reflective micro-machined silicon surfaces. The tuning assembly for this embodiment may comprise, for example, electrodes associated with the reflective surfaces of the air gap. The optical thickness of the air gap etalon is controlled by positioning one or both reflective surfaces according to voltage applied to the electrodes to vary the optical path length of the air gap. One or more of the electrodes are operatively coupled to the split detector, and the potential applied to the electrodes for control of the air gap spacing is responsive to the error signal derived from the split detector.
In another embodiment of the invention the tunable element comprises an electro-optic device having an effective optical path length or optical thickness that is adjustable according to an applied electric field, magnetic field, mechanical stress via gas pressure or other source, thermal, or non-linear optical effect. The electro-optic device may comprise, for example a substrate made of a electro-optic material such as a liquid crystalline material wherein the refractive index of the substrate can be varied by suitable application of voltage to the substrate. The tuning assembly in this case comprises electrodes associated with the electro-optic material substrate which are suitably positioned to control the refractive index of the etalon electro-optic substrate material.
In another embodiment the tunable element may comprise an air gap etalon defined by reflective surfaces which are movable via a piezoelectric material associated with the reflective surfaces wherein the air gap separation can be varied by suitable application of voltage to the piezoelectric material. The tunable element may comprise an air gap etalon with reflective surfaces that are movable via thermal control, using thermal expansion and contraction via heating and cooling of a spacer associated with the reflective surfaces to provide tuning.
In another embodiment of the invention the tunable element comprises a grating or like retroreflective element positioned in the coherent light beam. The constructive interference fringe in this embodiment is defined by the grating spacing and angular relationship of the grating and light beam. The tuning assembly comprises a mechanical, electrical, piezoelectric or like system configured to rotate or otherwise control the angle of the grating with respect to the light beam according to the error signal from the detector. The detector may be positioned for detection of light transmitted through or reflected from the grating. The grating may be chirped and configured for near field detection by the split detector. The grating may be un-chirped and the detector is positioned for far field detection, with suitable collimating optics positioned to direct light from the grating to the split detector. The grating may be positioned in an external cavity laser in a Littrow or Litman-Metcalf configuration, or other configuration.
The error signal generation system of the invention may be embodied in an external cavity laser apparatus wherein the tunable element is located within or otherwise associated with the external cavity and is positionable to provide selected transmission channel wavelengths. The external cavity laser will comprise a gain medium emitting a coherent light beam along an optical path, and an end mirror positioned in the optical path. The end mirror and a rear reflective facet of the gain medium define the external laser cavity. The gain medium may comprises an emitter chip which emits the coherent light beam along an optical path, with the tunable element located within the cavity and positioned in the optical path. The detector may be positioned in the optical path after the end mirror. A tuning assembly is operatively coupled to the tunable element and to the detector.
A grid generator element is included in association with the external cavity laser for wavelength locking and is positioned in the optical path between the gain medium and end mirror. The grid generator element may comprise, for example, a grid etalon having a free spectral range corresponding to the spacing between the gridlines of a selected wavelength grid such as the ITU wavelength grid. The tunable element may, in one embodiment, comprise a wedge etalon which is positioned in the optical path within the external cavity laser between the gain medium and end mirror, so that the grid etalon is positioned between the gain medium and wedge etalon, and the wedge etalon is positioned between the grid etalon and end mirror, with the light beam passing through the grid etalon and wedge etalon along the optical path. Transmission wavelength channel selection according to the grid defined by the grid etalon is provided by positional adjustment of the tunable element and/or end mirror. Positioning of the wedge etalon for channel selection is carried out by driving the wedge to selected or appropriate positions wherein the optical thickness of the wedge etalon corresponds to an integral multiple of the half wavelength for the selected channels.
The wedge etalon acts as an interference filter, with the tapered shape of the etalon defining a constructive interference fringe, as noted above. In order to avoid spatial losses to the beam passing through the wedge etalon, the constructive interference fringe defined by the wedge etalon must be centered in the optical path defined by the beam. The detector, which receives the beam passing through the wedge etalon, is configured to generate or provide a difference error signal indicative of the position of the constructive interference fringe in the beam, and hence any spatial losses to the beam associated with the position of the constructive interference fringe. The detector may be a split detector or other multi-element detector, a lateral effect detector, or other type of detector.
When the constructive interference fringe is centered in the optical path, the optical power received by the two halves of the split detector will be equal, and the difference error signal derived from the detector will nominally be zero. When the constructive interference fringe is not properly centered, the resulting spatial losses to the beam result in a beam spot on the split detector that is truncated, such that the two detector halves receive different levels of optical power, and a non-zero difference error signal results. The tuning assembly re-positions the wedge etalon according to the error signal so that the constructive interference fringe is centered in the beam. The position of the constructive interference fringe defined by the wedge etalon is continuously adjusted according to error signals from the split detector to maintain the constructive interference fringe in the center of the beam and avoid spatial losses associated with non-optimal positioning of the wedge etalon. The positional adjustment of the wedge etalon in this manner also avoids unintended channel changing in the external cavity laser.
The tuning assembly used to adjust the position the wedge etalon according to the error signals derived from the detector may comprise the same mechanical drive or translation assembly used for channel selection with the wedge etalon. Thus, in the operation of the external cavity laser, channel selections will periodically be made by driving the wedge etalon to predetermined positions. Servoing the position of the wedge etalon is carried out continuously at each channel wavelength to avoid spatial losses to the beam.
In other embodiments, the tunable element used in the external cavity may alternatively comprise a grating, with the grating angle servoed according to the error signal derived from the split detector. In still other embodiments, the tunable element may comprise a liquid crystal or ferroelectric material-based etalon in which the effective optical pathlength through the etalon is servoed via voltage controlled refractive index changes in the etalon material according to the error signals. Various components of the external cavity laser may be embodied in a MEMS device, with the tunable element comprising an air gap etalon with an optical path length adjustable by voltage controlled movement of a reflective surface, as also noted above.
The use of a split detector with an external cavity laser as provided by the invention also allows servoing of other optical components of the external cavity laser to error signals derived from the split detector. An end mirror tuning assembly may be included in association with the end mirror, and may comprises an oscillator element and a translator element. The translator may comprise an arm which is thermally positionable via a thermoelectric controller and thermistor, while the oscillator element comprises a piezoelectric element configured to periodically oscillate the end mirror. The thermoelectric controller is operatively coupled to the split detector via a sum signal generator and a phase compensator, and operates to thermally move the translator arm according to a phase corrected sum signal derived from the split detector. The oscillation of the end mirror via the piezoelectric element creates frequency shifts which allow tracking of amplitude modulation for servoing the end mirror. The drive current provided to the gain medium may also be servoed according to error signals derived from the split detector.
As will be apparent from the following detailed description, the invention provides an error signal generation system which prevents spatial losses in coherent optical systems, which generates error signals indicative of spatial losses associated with a coherent light beam, which provides for adjustment of a constructive interference fringe associated with a tunable element according error signals derived from a split detector, which is usable with a continuously tunable external cavity laser, and which allows adjustment of multiple optical components associated with an external cavity laser according to error signals derived from a single split detector. The error signal generation system of the invention can further be used for spectrally filtering out spontaneous emission light associated with a coherent light source.